Having their basis in bug and plant prevalence and weather conditions, many of the skin problems that plague horses are seasonal. This month, with the help of two veterinary dermatologists--one based in Texas, the other in Canada--we'll offer ways to recognize and respond to five skin ailments likely to appear in spring or early summer: insect hypersensitivity, sweet itch, contact dermatitis, scratches, and rain rot. Then in October we'll do the same for problems typically arising in cooler weather.

Insect Hypersensitivity

As spring temperatures warm, several biting insect species--black flies, horn flies, stable flies, mosquitoes, and (of course) horseflies--hatch, proliferate, and seek nourishment. "Insect hypersensitivity is the most common equine allergy," says Christine Rees, DVM, Dipl. ACVD, who taught for 13 years at Texas A&M's veterinary school before opening an equine/small animal dermatology practice in Dallas. More specifically, Ontario Veterinary College associate professor of dermatology Anthony Yu, DVM, MS, Dipl. ACVD, designates spring and summer insect hypersensitivity as insect-bite hypersensitivity, in which the bite introduces saliva that allows the insect to access more blood flow and sometimes triggers an allergic reaction.

Reactions can include severe itching with hair loss and crusting around the mane, tail, and trunk of the body, granulomas (chronic inflammatory lesions), and even heaves, an allergic respiratory disease. But the typical allergic reaction to these insect bites is hives--multiple bumps of fairly uniform size (from less than a quarter-inch to more than an inch across), usually covering an area of the body or neck. Larger hives might "weep" serum, matting the coat and causing sores; smaller ones might just make the skin feel coarse to the touch or cause a patch of hair coat to look rough. (However, hives aren't always caused by bug bites; see sidebar on page 25.)

While some hives don't cause discomfort, typically they are itchy enough that the horse tries to obtain relief by rubbing the affected area. Reese notes this discomfort might bother the horse more than the owner realizes. "Look at body language," she says. "If you touch the affected area, he may wince or otherwise show discomfort." She urges owners not to ride a horse while the condition persists, saying, "I figure that if my skin looked like that, it would bother me."

The best approach to hypersensitivity is prevention: Minimize insects' access to horses by means of barn cleanliness, ventilation, and screens; eliminate standing water where insects breed and larvae can incubate; keep horses in the barn when biting insects are most active; and use protective turnout sheets and fly masks. An additional option, Rees says, is administering a feed-through "insect-growth regulator" that prevents fly larvae in manure from reaching maturity. "Every so often, switch out whatever fly spray you're using because some insects seem to build resistance to a single product over time," she adds. Aside from repelling insects, "Permethrin and permethrin derivatives also soothe; oil-based medications like these stay on the skin longer and so work a little better."

If these measures don't provide relief, your veterinarian might prescribe systemic medication or recommend testing to confirm the responsible allergen(s). "In a few cases, what looks like an insect allergy is actually an environmental allergy," notes Rees. "Particularly if I know an owner is being conscientious with insect-control measures that apparently aren't being effective, I'll do a skin test; I may discover that the horse is allergic to some weeds or grasses or pollen."

The Many Causes of Hives

Hives are a reaction--but to what? "The list of underlying causes is very long," says Christine Rees, DVM, Dipl. ACVD, who taught for 13 years at Texas A&M's veterinary school before opening an equine/small animal dermatology practice in Dallas. "In horses, insect hypersensitivity is the most common. But there are also contact allergies and environmental allergies that cause hives." And they can appear "in response to a physical factor, such as exercise or heat or cold or tension."

Ontario Veterinary College associate professor of dermatology, Anthony Yu, DVM, MS, Dipl. ACVD, points out that "a combination of things can be what pushes an individual over his allergic threshold--for example, there's molasses or alfalfa in the horse's feed, plus he's being fed carrots and apples, plus he's been out in the pasture at the time when the birch trees are pollinating. All these things have proteins that can be absorbed through his oral mucosa, skin, or via inhalation. They may all add up and push the horse over to the point that he starts developing a hivelike reaction."

So how do you provide relief? Yu uses the metaphor of fighting a forest fire: You don't just want to smother discomfort-causing inflammation--you also must prevent it from spreading.

"Steroids are our best tools for 'putting out the fire' because they provide the quickest response (by reducing inflammation). Then, to prevent further hives, we use medications such as antihistamines while letting the existing reactions subside on their own, possibly along with supplements containing essential fatty acids, and perhaps a medication called pentoxifylline (Trental). With these medications we're trying to dig a trench around the fire and prevent it from spreading.

"A multimodal approach is often necessary to control hives," Yu continues. "If, however, hives flare up again to a point of possibly being able to 'jump the trench'--they're no longer controlled by the other medications. That's when we 'call in the planes to dump the fire retardant': give steroids again. But we don't want to bring that plane around often, because the retardant itself can have some detrimental effects (e.g., laminitis and recurrent infections)."

--D.J. Carey Lyons

Sweet Itch

Also called "Queensland itch" or "summer eczema," sweet itch is "a reaction to salivary antigens in the bite of the Culicoides gnat," says Yu.

In affected horses, small pimplelike papules erupt on the skin, making hair stand erect and causing severe itching. The mane and tailhead are particularly common sites, and the horse might rub them so intensely that they become practically hairless or develop ulceration and scabbing. Testing for antigens confirms the diagnosis; however, Yu notes, "There are different species of Culicoides throughout the world. (For accurate diagnosis and treatment), it's very important to test for the particular species that's endemic to the horse's region."

Again, preventing sweet itch is the first goal; measures that help minimize general insect-bite risks help defend against Culicoides. But because the gnats are extremely small, only fine-mesh screens (32-by-32 holes or more per square inch) effectively prevent their passage.

Relief, as with other insect-bite hypersensitivity, starts with topical corticosteroids, followed by systemic antihistamines. Again, supplements with eicosapentanoic acid can reduce allergic reactions, says Rees, who has also found some benefit treating sweet itch "and possibly other insect allergies" with supplements containing MSM (methylsulfonylmethane, an anti-inflammatory often used to treat joints).

As the name indicates, contact dermatitis is inflammation resulting from some substance making contact with the skin. The challenge is figuring out which substance, because something that causes one horse profound irritation might not bother another.

Location is one clue to what's going on. "If the problem is a blanket," says Rees, "it's pretty obvious, because the lesions are on the back. If it's a rubber bit, you'll see redness around the lips or muzzle. If it's all over the body and you routinely shampoo the horse, think about the shampoo. If it's localized around the coronary band or the lower leg, it could be insects (or plants)--but especially if it's around the hoof and it's red and inflamed, think about allergy to bedding. Some horses are allergic to a particular kind of wood; shavings from that wood can give them problems." History is the other diagnostic clue: What's changed lately in the horse's routine, surroundings, equipment, shampoos, sprays, medications, or even diet?

Again, topical corticosteroids can relieve initial inflammation. Then start your detective work: Look at where the irritation is; think back over your horse's history; then start removing possible irritants, one at a time. After each change, clean the inflamed area with mild soap and water; then wait a few days. Contact dermatitis will reappear until you eliminate the cause.

Scratches

Scratches is a nonspecific diagnosis similar to hives in that a number of things can cause the condition, says Rees. "In some cases the condition is fungal; in some it's bacterial, or a contact dermatitis, or an insect allergy," she explains. Thus, Yu, at the minimum, recommends veterinarians perform diagnostic skin scrapings and cytology (study of the cells) and perhaps a fungal culture.

Characteristics typical of scratches--also called "pastern dermatitis" or "greasy heel"--include infection, inflammation, and swelling at the rear of the pasterns and possibly farther up the legs; crusty, scabby, or weepy lesions; skin that's red and painful to the touch; and hair loss.

When treating scratches, Rees says to clip excess pastern hair if the horse tolerates it. Then wash the area gently--since it might be quite painful--with an antiseptic soap such as Betadine surgical scrub or chlorhexidine solution. Leave the scrub on for 10 minutes before rinsing and drying well. Apply antibiotic ointment to the lesions. For severe infections consult your veterinarian, who might prescribe systemic antimicrobials.

Rain Rot

As the name suggests, rain rot--or dermatophilosis, caused by Dermatophilus congolensis bacteria--is a rainy-season problem. So if spring is wet, that's when it will probably show up; if fall or winter is wet, it's likely then as well. "The Dermatophilus organism needs moisture to proliferate, release, and spread the infective forms of the bacteria, called zoospores. Horses might become infected from direct contact with contaminated fomites (inanimate objects or substances capable of carrying infectious organisms; for example, the bacterium can survive dormant in bits of fallen-off scab for months) or an asymptomatic carrier, or indirectly through transmission by insects," Yu explains. "(Rain rot) needs a triple combination of circumstances, moisture plus bacteria plus trauma, to cause clinical signs. Eliminate any one of those three factors and you eliminate the development of the disease. The 'trauma' could be induced by an insect carrying the organism and whose bite damages the epidermal barrier of the skin, creating an opening for the bacteria to get in. Rain (or moisture in general) is the last necessary ingredient to make it an active disease."

Rain rot proliferates along the back--from neck and withers to croup--as well as on the legs. On the horse's skin, explains Rees, it causes kind of a crust; when the crust lifts up, the skin beneath is ulcerated. In a long winter coat, raised tufts of serum-matted hair, called paintbrush lesions, develop; they might get pulled out or brushed off. Itching does not seem to be a problem, but touching affected skin or picking at the scabs can cause pain.

Remember that rain rot can spread between horses. Thus, isolate any affected horse; don't share tack, equipment, or blankets; be certain the infected horse is always the last one you handle; and after treating the animal wash your hands and clean the treatment area (e.g., wash rack or grooming stall). To relieve discomfort, "injected or oral antibiotics may be needed," Rees says, "but if the condition isn't too severe, you may be able to deal with it through a topical treatment, such as a peroxide shampoo." Then to avoid enablingbacteria growth, dry the area completely.

Take-Home Message

The best offense against warm-season equine skin problems is a savvy defense: eliminating risks and irritants, minimizing horses' exposure to bugs and bacteria, checking scrupulously for bumps and rubs and sensitive spots every time you groom, using history and observation to deduce probable causes for those itches or ouches, and teaming up with your veterinarian to help your horses fight allergens.

In October we will show how to mount a similarly effective defense against a different set of problems: those for which risk rises as the average temperature drops, winter hair coats sprout, and horses spend more time blanketed and/or indoors.